The present invention relates to a color image forming apparatus, particularly to an image processing apparatus for performing proper color reproduction of a digital color copying machine.
In a conventional digital color copying machine, a color image of an original is subjected to color separation into three colors of red (R), green (G), and blue (B) for each pixel, reflectance is read, and the image is converted to digital image data of three colors R, G, B. The three-color digital image data is subjected to a color correction processing such as a density conversion processing, undercolor removal processing, black ink addition processing, and masking processing, and four-color digital image density data of cyan (C), magenta (M), yellow (Y), black (K) is obtained.
A dot image as a digital image is recorded on a copy sheet based on the four-color digital image density data.
In the black ink addition processing, a black toner having a predetermined black ink addition amount is added to the other three-color toners C, M, Y, density is replenished in a high density region, and reproducibility in a shadow portion of the image can be enhanced. Moreover, in the undercolor removal processing, a predetermined undercolor removal amount of a gray component is removed from the other three-color toners, and replaced with the black toner, so that color registration is strengthened, and reproducibility of a character, and the like can be enhanced.
Moreover, since respective optimum values of the black ink addition amount of the black ink addition processing and the undercolor removal amount of the undercolor removal processing differ depending upon the type of the original image to be formed. Therefore, there is proposed an apparatus for inputting the type of the original, such as a photograph, text, and the like and changing the black ink addition amount and undercolor removal amount respectively.
Furthermore, in Jpn. Pat. Appln. KOKAI Publication No. 4-200271, the black ink addition amount and undercolor removal amount are changed in accordance with a judgment result of white/black or color.
Additionally, in Jpn. Pat. Appln. KOKAI Publication No. 4-341061, the black ink addition amount and undercolor removal amount are changed in accordance with the judgment result (4-stage evaluation) of a chromatic color or an achromatic color.
Moreover, in Jpn. Pat. Appln. KOKAI Publication No. 5-14696, the black ink addition amount and undercolor removal amount are determined in accordance with chroma of HVC conversion for conversion to data of hue (H), value (V), chroma (C).
However, even in the aforementioned method, it is difficult to allow proper gray reproduction to be compatible with high chroma color representation, it is also difficult to allow proper black reproduction of a photograph portion to be compatible with proper reproduction of a black character, and further there is a problem that dark color reproduction becomes unstable.